The technical mechanism of conventional bridge plugging for improving the bearing capacity of the formation usually is to composite a variety of bridge plugging materials into plugging slurry; utilize high pressure to press the plugging slurry to leakage loss positions in the formation, so that the bridge plugging materials adhere to, bridge up, and pack up the pore throats and form a packing layer that will not be pushed away because the packing layer has high friction with the wall surfaces of the fractures or pores; and then utilize the thin and smooth flake materials that may be curved and deformed easily in the plugging slurry to pack up the pores and utilize plant fibers to brace and link up the materials to form a dense packing layer, so as to attain the purposes of plug up the pores, eliminate leakage loss, and improve the bearing capacity of the formation. The functional mechanism includes the following aspects:
(1) Throat plugging effect: the bridge plugging materials utilize their volume to form bridges at the throats in fractures (pores), and thereby turn fractures into pores and turn large pores into small pores;
(2) Packing effect: the granules in different sizes pack up the remaining spaces, so as to reduce and eliminate the leakage loss channels and thereby attain the purpose of eliminating well leakage;
(3) Infiltration effect: since many bridge plugging materials that may cause high filtrate loss are added in the drilling fluid, the drilling fluid may have high filtrate loss easily and form thick filter cakes, which enhance the plugging effect;
(4) Bracing and linking effect: the fibrous and flaky plugging materials attain a bracing and linking effect, and thereby brace and link up the packing layer firmly and greatly increase the strength of the filter cakes; as a result, the resultant packing layer is hard to move in the leakage fractures and the firmness of the packing layer is increased;(5) Water swelling effect: some of the bridge plugging materials have certain water swelling capability; when those materials are pressed into the formation fractures and form a bridge bedding layer, they are soaked in the liquid in the formation and thereby swell; thus, the plugging capability of the bridge bedding layer is enhanced.
Conventional bridge plugging materials are mainly inert materials, and may be in granular shape, flake shape, and strip shape, etc. Common granular materials include walnut shells and rubber granules, etc., they plug the “throats” of leakage channels in the plugging process and attain a “bridging” effect, and are also referred to as “bridging agents” accordingly; strip materials are from plants, animals, minerals, and a series of synthetic fiber materials, such as sawdust and asbestos, etc., they attain a suspension effect in the plugging slurry and interlace and brace each other in the plugging layer, and are referred to as “suspending and bracing agents” accordingly; flaky materials include vermiculite and mica sheets, etc., they mainly attain a packing effect in the plugging process, and are referred to as “packing agents” accordingly.
There is no uniform specification for the granules size of conventional bridge plugging materials. Usually, granular materials are classified into three grades, i.e., coarse, medium, and fine granules, wherein, coarse granules are within the range of 4-10 meshes, medium granules are within the range of 10-20 meshes, and fine granules are higher than 20 meshes. Flaky materials usually shall be sieved through a 4-mesh sieve to prevent them from sealing up the bit port. Flexible flaky materials may be in a size as high as 25.4 mm, Flaky materials shall have certain water resistance performance; the strength of a flaky material shall not be decreased by a half or more after the flaky material is soaked in water for 24 h; flaky materials shall not be broken after they are folded repeatedly at the same position; flaky materials with high flexibility may have thickness up to 0.25 mm.
A composite bridge plugging material is a plugging material prepared by compositing several bridge plugging materials at a certain ratio. Common composite plugging materials including rubber granular composite plugging agent, FDJ composite plugging agent, HD composite plugging agent, and cotton seed hull plugging agent, etc. The rubber composite plugging agent is a new bridge plugging agent, and is composited from 35% rubber granules, 20% walnut shells, 15% oyster shell powder, 10% sawdust, 12% cotton seed hulls, 5% peanut hulls, and 3% rice straws. The FDJ composite plugging agent is a composite plugging agent developed utilizing inert rigid plugging materials as the main materials, inorganic salts as reinforcing agents, and polymers as assistant agents. The HD composite plugging agent is composited from walnut shells, mica, rubber, vermiculite, cottonseed hulls, sawdust, and coir fibers at a ratio of 3:2:3:2:1:1:0.1. The cotton seed hull plugging agent is a granular composite plugging agent composited from cotton seed hull powder, cotton seed hull granules, bentonite, asbestos, and surfactant. After the plugging agent is pressed into the formation, it begins to absorb water in a large quantity after a while, and thereby swells and breaks, accumulates at the pore throats, and seals the leakage channels, so that the purpose of plugging is attained.
However, the above-mentioned conventional bridge plugging technique has a series of problems, including instable performance and low bearing capacity, etc., when it is used in deep well or extra-deep well applications. Consequently, once a serious leakage loss situation is encountered during the drilling of a deep well or extra-deep well, the difficulty in handling the situation will be increased, and the cost will be severely increased. Therefore, it is urgent task to see for a plugging agent that is suitable for plugging against serious leakage loss in deep wells or extra-deep wells.